Southern California Two Color EDM Networks
Figure. This map shows the locations of
of 4 two-color EDM (Electronic Distance Meter) networks in Southern California.
These networks are radial with approximately 12 baselines using
a common, central station. Since near surface, aseismic slip is not apparent,
these radial networks are ideal for measuring the 3 components
of the tensor strain. The precision of these measurements is 0.1 part-per-million, but localized wobble of the geodetic monuments limit
the ultimate precision in our estimate of strain over long periods.
Measurements at Pinon Flat and Pearblossom are made approximately 3 to 4
time per year but the measurements at Anza and at the Buttes are made
approximately once per year. Each measurement of strain requires a 3 to
4 person crew to be on-site to run the instrument and set-up the reflectors
to make distance measurements on approximately a dozen baselines. Baseline
length are range between 2 and 10 km.
Maps of the network
The maps of the networks show the location of the baselines in red, the major faults with
dark, heavy lines, and other faults with thin lines.
- Map of the Pearblossom and Buttes networks.
- Map of the Pino Flat and Anza networks.
Plots of the data
- Plot of strain changes from the Pearblossom network.
- Plot of strain changes from the Buttes network.
- Plot of strain changes from the Pinon Flat network.
- Plot of strain changes from the Anza network.
Notes on data plots
In general, the vertical line denotes the time of the 1992 Landers
Earthquake.
The error bars represent one standard deviation
about the estimate of strain change. The error accounts for the
instrument precision of 0.1ppm and the random walk motions of each
geodetic monument of approximately 1 mm/sqrt(yr). The date of the
Landers earthquake is specified as a reference date and the error grows
in time relative to this date.
The secular rates have been removed from the strain data and the
rates are noted on the plot.
PEARBLOSSOM: Data starts in late 1980 when measurements were made
several times each week. This network straddles the San Andreas fault.
The strain changes estimated for the first
several years assume that strain changes linearly with time over 2-week
intervals. However, since 1985, measurements of the 12 baseline network
have been made approximately 3 to 4 times each year. The strain changes
that are plotted are reckoned such that parallel strain is extension
parallel with the local strike of the San Andreas fault, N65W, normal
strain is extension N25E, and shear strain is the tensor shear on a
plane parallel with the San Andreas fault. Because the network is
highly redundant for estimating the 3 components of the strain tensor,
the displacement of the central monument is also estimated as a
function of time.
BUTTES: Measurements of this network located north of Pearblossom
by 40 Km started in 1991. The strain changes that are plotted are
reckoned such that parallel strain is extension parallel with the local
strike of the San Andreas fault, N65W, normal strain is extension N25E,
and shear strain is the tensor shear on a plane parallel with the San
Andreas fault. Because the network is highly redundant for estimating
the 3 components of the strain tensor, the displacement of the central
monument is also estimated as a function of time.
PINON FLAT: Measurements of this network started in 1986. The
strain changes that are plotted are reckoned such that Eee is extension
in the east-west direction, Enn is extension in the north-south
direction, and Een is tensor shear on either a north-south or east-west
plane. Because this network is more-or-less a fan-shaped array rather
than a radial network, it is difficult to resolve the contribution of
wobble of the central monument from that of changes in strain. So, the
results of the strain analysis neglects the motion of the central
monument.
ANZA: Measurements of this network that straddles the San
Jacinto fault starts in 1988. The strain changes that are plotted
are reckoned such that Eee is extension in the east-west direction, Enn
is extension in the north-south direction, and Een is tensor shear on
either a north-south or east-west plane. Since this network consists of
two, radial networks, each with seven baselines, it is difficult to
resolve the contribution of wobble of the central monument from that of
changes in strain. So, the results of the strain analysis neglects the
motion of the central monument.
Secular strain rates
The changes in length of each baseline from each network can be
adequately modeled using spatially and temporally uniform strain. The
exception to the uniformity in rate of strain are the large, co-seismic
changes from the Landers Earthquake in 1992. Tabulated below are the
estimates of secular strain and co-seismic strain where all parameters
are estimated simultaneously from the data. The errors in secular rate
include an estimate of random-walk noise due to wobble of the geodetic
monuments.
Co-seismic strain change from Landers
For more details or to obtain the raw data, contact:
- John Langbein (langbein@usgs.gov)